Is a train moving 120kmh quick enough to kill you? ?

yes. One moving at 10 km/hr can kill you under the right circumstances.

With collisions you need to consider momentum, impulse and ultimately force.

120 km/h => 33.3 m/s

If the train hits you when you are at rest, then your change in momentum is mv => 80(33.3)

Note: We assume your mass is roughly 80kg

Now, Impulse => change in momentum

i.e. Impulse = 80(33.3) => 2666.7 Ns

or, Ft = 2666.7

So, if the collision takes roughly a quarter of a second we have:

F(0.25) = 2666.7

i.e. F = 10666.7 Newtons

You decide, but don't put it to the test!!

:)>

A train moving at such speed has thousand times the energy necessary for killing you

It's not just the speed you have to worry about. Better a car hitting you at that speed than a train weighing 250x as much. But even a car at that speed has been enough to kill people, less even.